We had ADR's months ago for three (2 Adults 1 toddler) and wanted to add 2 more adults, we had to accept our seating times changed by up to 30 minutes on a couple ressie's but none were impossible. Obviously we had to call to avoid losing our reservation, cancelling & remaking and losing the res# which can be a pain to keep track of when you have to change many of these 12 digit numbers. Places like character meals are harder to accommodate since an extra person may mean, chairs blocking the aisle and characters being spread more thinly. Some of those costumes are huge and can barely get past regular seating. Another thing to consider is they don't want an embarrassing scene at the restaurant so they are more accommodating in-person.